1. Field of the Invention
The invention relates to a method for the production of a silicon single crystal by pulling the single crystal, according to the Czochralski method, from a melt which is held in a rotating crucible, with the single crystal growing at a growth front. The invention also relates to a silicon single crystal and to semiconductor wafers which are separated therefrom.
2. The Prior Art
The production of single crystals which have a diameter of 200 mm or more represents a significant challenge, particularly since it is very difficult to deliberately adjust the radial crystal properties within a very narrow tolerance range. This applies to the concentration of impurities or dopants, and especially to the crystal defects and self-point defects, and agglomerates thereof. Self-point defects include interstitial silicon atoms (silicon self-interstitials) and vacancies, which are formed at the growth front of the single crystal. They very substantially determine the radial and axial defect distributions occurring later in the single crystal, and they also affect the impurity distributions which occur. For example, vacancies contribute to the precipitation of oxygen. When they exceed a size of about 70 nm, oxygen precipitates form oxygen-induced stacking faults (OSFs). The vacancies themselves can combine into agglomerates and form so-called COPs (crystal originated particles). Agglomerates of interstitial atoms form local crystal dislocations, which are also referred to as LPITs (large etch pits) because of the detection method which is used. The oxygen concentrations and the thermal conditions at the growth front and in the solidifying single crystal determine the nature and distribution of the crystal defects and impurities.
The thermal conditions when pulling the single crystal are dictated by the heat sources, i.e. the heating elements which are used, and the heat of crystallization released during solidification. The heat energy is transferred to the single crystal by radiation, heat conduction and heat transport, for example via the flows in the melt. The removal of heat in the vicinity of the growth front is determined to a large extent by the heat radiated from the edge of the single crystal and by the thermal dissipation in the single crystal. Overall, the thermal budget can be affected by the design of the pulling system, i.e. via the geometrical arrangement of the thermally conductive parts, the heat shields and by additional heat sources. The process conditions, for example growth rate, pressure, quantity, type and flow of inert gases through the pulling system furthermore contribute substantially to the thermal balance. Increasing the pressure or the quantity of inert gases, for example, will reduce the temperature. Faster pull rates produce more heat of crystallization.
The flows which transport heat in the melt are extremely difficult to predict. The heating elements, generally arranged in a ring around the crucible, produce a convective flow in the melt. Together with the counter-rotation conventionally used for the single crystal and crucible, this leads to pattern of movement in the melt which is distinguished by an upward melt flow at the edge of the crucible and a downward melt flow below the growing single crystal.
As experiments have shown, the movement of the melt also depends on the degree and direction of the rotation of the crucible and the single crystal. For example, iso-rotation and counter-rotation produce very different convection patterns. Crystal pulling with iso-rotation has already been studied (Zulehner/Huber in Crystals 8, Springer Verlag, Berlin Heidelberg 1982, pp 44-46). Counter-rotation is generally preferred because, compared to iso-rotation, it leads to a less oxygen-rich material and significantly more stable conditions for the crystal growth. The iso-rotation variant is not generally used on an industrial scale.
The flows which transport heat and oxygen in the melt can also be affected by the forces due to applied electromagnetic fields. Static or dynamic fields make it possible to alter the degree and direction of the flows in the melt, so that different oxygen contents can be obtained. They are primarily used for oxygen control. Magnetic fields are used in a number of variants, for example in the form of static magnetic fields (horizontal, vertical and CUSP magnetic fields), single-phase or polyphase alternating fields, rotating magnetic fields and traveling magnetic fields. According to U.S. Patent Application No. 2002/0092461 A1, for example, a traveling magnetic field is used in order to control the incorporation of oxygen into the single crystal. Recent numerical simulations for the effect of magnetic fields on the movement of the melt are presented, for example, in “Numerical investigation of silicon melt flow in large diameter CZ-crystal growth under the influence of steady and dynamic magnetic fields”, Journal of Crystal Growth, 230 (2001) 92-99.
The radial temperature distribution at the growth front of the crystal is extremely important for the crystal properties. It is determined essentially by the heat radiated from the edge of the single crystal. As a rule, a much more pronounced temperature drop is therefore observed at the edge of the single crystal than at its center. The axial temperature drop is usually denoted by G (axial temperature gradient). Its radial variation G®) very substantially determines the self-point defect distribution, and therefore the other crystal properties as well. The radial change of the temperature gradient G due to the thermal budget is generally determined by numerical simulation calculations. The radial variation of the temperature gradient can be experimentally determined from the behavior of the radial crystal defect distribution for different growth rates.
The ratio V/G®) is of crucial importance in terms of the creation of crystal defects, G®) being the axial temperature gradient at the growth front of the single crystal and depending on the radial position (the radius r) in the single crystal, and V being the rate at which the single crystal is pulled from the melt. If the ratio V/G is more than a critical value k1, then vacancy defects (vacancies) predominantly occur; these can agglomerate and then be identified, for example, as COPs (crystal originated particles). Depending on the detection method, they are sometimes referred to as LPDs (light point defects) or LLSs (localized light scatterers). Because of the usually decreasing radial profile of V/G, the largest COPs most commonly occur at the center of the crystal. They generally have a diameter of about 100 nm, and therefore cause problems for component fabrication. The size and number of the COPs is determined by the initial concentrations of the vacancies, the cooling rate and the presence of impurities during agglomeration. For example, the presence of nitrogen leads to a shift of the size distribution toward smaller COPs with a larger defect density.
If the ratio V/G is lower than a critical value k2, which is less than k1, then self-point defects are predominantly found in the form of interstitial atoms (silicon self-interstitials), which can also produce agglomerates and are microscopically seen as dislocation loops. These are often referred to as A swirls, and the smaller form as B swirls, or as LPIT defects (large etch pits) for short because of their appearance. The size of LPITs lies in the range up to 10 μm. As a rule, not even epitaxial layers can cover up these defects perfectly. These defects as well can also impair the functionality of the electronic components fabricated on silicon wafers.
In the broadest sense, the region in which neither agglomeration of vacancies nor agglomeration of interstitial atoms takes place, i.e. in which V/G lies between k1 and k2, is referred to as a neutral zone or perfect region. The value of V/G at which the crystal changes from excess vacancies to excess interstitials naturally lies between k1 and k2, and is given in the literature as the critical limit Ccrit=1.3*10−3 cm2 min−1 K−1 (Ammon, Journal of Crystal Growth, 151, 1995, 273-277). In a more specific sense, however, distinction is also made between a region in which there are still free unagglomerated vacancies and a particular region of free interstitial atoms. The vacancy region, also referred to as the v region (vacancies), is distinguished in that if the oxygen content of the single crystal is high enough, oxidation-induced stacking faults are created there, while the I region (interstitials) remains fully fault-free. In this more specific sense, therefore, only the I region is actually a perfect crystal region.
Large ingrown oxygen precipitates with a diameter of more than about 70 nm can be revealed as oxygen-induced stacking faults (OSFs). To this end, the semiconductor wafers cut from the single crystal are subjected to a special heat treatment, which is referred to as wet oxidation. The growth rate of the oxygen precipitates created during the crystal pulling, which are sometimes also referred to as grown BMDs (bulk micro-defects), is promoted by vacancies in the silicon lattice. OSFs are therefore encountered primarily in the v region.
The single crystal would be virtually defect-free if the pulling conditions can be adjusted so that the radial profile of the defect function V/G®) lies within the critical limits for COP or LPIT formation. This is not easy to achieve, however, especially when single crystals with a comparatively large diameter are being pulled, because the value of G then depends significantly on the radial position r. In general, owing to the radiative heat losses, the temperature gradient G is very much greater at the edge of the single crystal than at the center.
The radial profile of the defect function V/G®), or of the temperature gradient G®), can lead to there being several defect regions on a semiconductor wafer cut from a single crystal. COPS preferentially occur at the center. The size distribution of the agglomerated vacancies is dictated by the cooling rate of the single crystal in the vicinity of the growth front. The size distribution of the COPs can be altered from a few large COPs to many small, less perturbing COPs by a high cooling rate (more than 2 K/min), or short dwell times in the temperature range from the melting point to about 1100° C., or by doping the melt with nitrogen. Furthermore, a radial size distribution such that smaller defects are formed with increasing radius is found in the COP region. The COP region is followed by an oxygen-induced stacking fault ring (OSF), due to the interaction of vacancies and oxygen precipitates. Outside this is a fully defect-free region, which is in turn bounded by a region with crystal defects consisting of interstitial agglomerates (LPITs). At the edge of the single crystal, the interstitial atoms diffuse as a function of the thermal conditions, so that a centimeter-wide defect-free ring may also be created there.
The crystal defect regions that occur have already been discussed at length, in relation to the radial V/G profile, by Eidenzon/Puzanov in Inorganic Materials, vol. 33, No. 3, 1997, pp 219-255. This article has already indicated possible ways of producing defect-free material. Both the cooling rate in temperature range during agglomeration, the effect of nitrogen doping and methods such as oscillating growth rate are referred to in this context.
To a certain extent, radial homogenization of V/G(r) can be achieved by using passive or active heat shields in the vicinity of the solidification front, as proposed for example in U.S. Pat. No. 6,153,008. Most publications relate to an effect on the cooling behavior due to modified heat shields. With the known prior art, however, sufficient radial V/G homogenization for the production of perfect silicon, especially with large crystal diameters, cannot be achieved in this way. By means of impurities, for example nitrogen or carbon, but also oxygen, the size and positioning of the defect distribution can be influenced so that the precipitation of impurities such as oxygen, can also be influenced. It is therefore of great importance to be able to deliberately produce and control both axial and radial impurity profiles.